This system is fully irreversible: airforces on the elevator push against the cylinder piston and are not fed back to the mechanical linkage at all. Other transfer functions are possible bit programmed feedback loops and transducers.
In this set-up, elevator deflection velocity is proportional to stick deflection. This mechanical feedback system is robust because there are very few electronics that can fail. Because of the movement of the piston, the servo valve closes again. The pilot input linkage is attached to the piston as well.The piston deflects because the servo valve is open.The pilot gives a control input, which opens the servo valve (the little box on top of the cylinder).The diagram is a bit primitive, but the gist of it is: You can see that the cylinder is anchored to the airplane structure. Here is a schematic diagram of a hydraulic actuation system with a mechanical servo valve: At first, hydromechanical designs, consisting of. These work in the same way as described above, with a follow up mechanism that moves the actuator until the valve closes, but the mechanical linkage is different. To make the control force required by pilots manageable, aircraft engineers designed more complex systems. You mention half powered controls in a comment - I assume you mean boosted flight controls, where the pilot directly deflects the control surface but hydraulics help in overcoming the aerodynamic forces. The actuator moves with it, in doing so closing the valve again.
This makes the flight control surface deflect. The pilot control stick is connected to the servo valve, and when he deflects the stick he opens the valve and oil starts to flow. Open loop servo valves just generate an actuator velocity, not very desirable when flying a plane.Ī more robust system used on aircraft has the hydraulic actuator moving together with the flight control surface. The loop uses position of the actuator as a feedback signal, measured by a position transducer, usually an LVDT. however, in the event of a complete failure of the aircraft’s flight-control-computer system, the a320 can switch to a backup mode known as mechanical law, which bypasses the fccs completely. There is a position control loop in there that opens the servo valve until the required position is reached, then closes it.
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